<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>

<input type="text" id="i1">
<input type="button" value="平方" onclick="f()">
<div>结果:<span></span></div>
<script>
    function f() {
        let i = document.querySelector("#i1");
        let s = document.querySelector("span");
        if(isNaN(i.value)){
            s.innerText= "输入错误！";
            return;
        }
        //当两个字符串进行-*/运算时会自动转成数值，进行+运算是拼接
        s.innerText = (i.value)*(i.value);
        //parseFloat()将字符串转成整数或小数
        // s.innerText = i.value*1+i.value*1;
        // s.innerText = parseFloat(i.value)+parseFloat(i.value);
    }
</script>
</body>
</html>